The present invention pertains to methods and apparatus for attaching the blade carrying wheels of a steam turbine to the turbine shaft.
For large steam turbines, it has not been entirely practical, at least for certain sections of the turbine, to fabricate the rotatable shaft and the wheels which carry the buckets, or blades, from an integral forging. For example, turbines deriving their power from nuclear energy sources often include low pressure sections which have wheel diameters on the order of six feet or more. Although a very limited number of forgings of this size, which permit the wheels to be machined integrally with the shaft, have been made, they are not readily available and larger diameter integral shaft and wheel combinations have not been demonstrated in commercial use.
What has generally been done instead, is to fabricate the wheels and shaft separately and then to shrink fit the wheels into position on the shaft. This has provided an acceptable manufacturing process. However, to further insure that the shrink fitted wheels do not, under any circumstances, rotate with respect to the shaft, antirotation keying means have been provided, in addition to the shrink fit, to further lock the wheels to the shaft. The antirotation keying means have, however, raised a separate set of problems.
In general, the wheels are keyed to the shaft by providing corresponding grooves or slots in the wheel bore and in the shaft surface. A separate metal block, appropriately termed a key, is inserted in the space defined by aligning the corresponding grooves. This further locks the wheel and shaft together. Problems with this configuration have resulted from the necessity of forming the keying grooves in an area of very high mechanical stress. For a shrunk-on turbine wheel, mechanical stress is greatest at the bore since it is the bore about which all of the rotating mass of the wheel is distributed. The keying means previously used, and as described above, have been located at the wheel bores, putting them at this point of intense stress. Because of surface discontinuities, inherent in the shape of the key and keyway these keying means have the potential for concentrating the stress even further. The result, in some turbine wheels, has been stress corrosion cracking in the area of the keyways to the point of destroying a wheel and putting the turbine out of service.
A separate problem with previous keying means has resulted from the fact that the keyway grooves have run from one side of the wheel to the other across a considerable temperature gradient. This had led to steam condensation in the grooves, the formation of liquid water therein, and errosion of the keyways by cavitation related processes.
In consideration of the shortcomings of previously used keying means for turbine shrunk-on wheels, it is an object of the present invention to provide an improved keying means which is subjected to substantially lower levels of mechanical stress than previously used wheel keying means and which is exposed to a substantially uniform operating temperature to prevent steam condensation and water errosion within the keying means.
It is a further object of the invention to provide means for keying shrink fitted turbine wheels to a shaft which avoids keys and keyways at the wheel bore, particularly for certain wheels which operate at steam conditions under which erosion and stress corrosion cracking may occur.
Still further objects and advantages of the invention will be apparent from an understanding of the principles and operation of the invention which will be gained from the detailed description of a preferred form of the invention which follows herein.